1. Field of the Invention
The invention relates to a large-capacity fiber optical switching device and an optical transmission system. For example, the invention relates to an optical switching device connected to multiple large-capacity optical fibers, each of which has multiple cores, and an optical transmission system using the optical switching device.
2. Description of the Related Arts
The approach for broadband through the optical fiber communication has brought about low-cost distribution of the large-capacity digital information. The new service using the approach further promotes the broadband communications, thus increasing the traffic volume of the internet at high rate twice as high as the one two years before. The optical fiber network has been developed, through which the large-capacity data are sent/received at high speeds for a relatively long distance of several kilometers, for example, backbone system, metro system, access system and the like. The optical interconnect technique for conversion of the signal wiring to fiber optics is thought to be effective for the short distance (several meters to several hundred meters) between the devices of the information communication Technology (ICT) system such as server of the data center, or the very short distance (several centimeters to several tens centimeters) in the device.
Meanwhile, research and development of communication capacity expansion of the optical fiber by means of such technique as wavelength division multiplex and multi-level modulation have been conducted with respect to the capacity increase. However, such study has reached the physical limit. The optical communication technique using the multicore fiber (hereinafter referred to as MCF) has been expected as the one for overcoming such limit. The generally employed single fiber only has one transmission channel through the single core. The multicore fiber has the transmission channel through multiple cores in the single fiber, which has drawn interest as the transmission medium that allows large-capacity high-density transmission. The research and development have been actively conducted in various organizations.
From the aforementioned circumstance, the study on the optical interconnect has been started for the purpose of realizing the large-capacity fiber such as the multicore fiber, and compact high density transmission. For example, Translation of PCT Application No. 2012-514768 discloses the optical interconnect technique configured to lay out the grating coupler in accordance with arrangement of the MCF cores using the photo diode (PD) via the waveguide or the optical device (optical chip) integrated for connection to the modulator. The optical interconnect in the device, and between the devices for realizing the compact high density transmission is thought to be indispensable for increase in the capacity of the data center. However, the use of the large-capacity high density wiring network may cause the risk of unexpectedly serious damage caused by the failure on the transmission path resulting from natural disasters and disconnection, thus needing the solution by which the high reliability is realized.
As the method for realizing the optical network with high reliability, an APS (Auto Protection Switch) method is known, which is employed for SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy), and OTN (Optical Transport Network) in the long-distance trunk system network. The redundant system such as 1+1, 1:N may be thought through the allocation method of active/standby system.
The 1:N redundant system configuration will be described referring to FIG. 1. The optical signal used for the SONET/SDH and OTN has a frame structure for transmitting both main signals and supervisory control information. Generally employed switching devices 1 and 2 are connected through n pairs of optical fibers 1000-1 to 1000-n, and 1001-1 to 1001-n. The switching device 1 is composed of a supervisory control unit 50, active signal transmission optical transceivers 70-1 to 70-n, and a standby signal transmission optical transceiver 80. The switching device 2 is composed of a supervisory control unit 51, active signal transmission optical transceivers 71-1 to 71-n, and a standby signal transmission optical transceiver 81. If, for example, the optical fiber 1000-1 fails to transmit the optical signal owing to disconnection, failure information detected by the active signal transmission optical transceiver 71-1 of the switching device 2 is carried on a supervisory control signal part of the transmission frame so as to be received by the active signal transmission optical transceiver 70-1 of the switching device 1 via the optical fiber 1001-1. Upon reception of the failure information through the optical fiber 1000-1, the supervisory control unit 50 of the switching device 1 and the supervisory control unit 51 of the switching device 2 are linked for signal switching so that the signal transmitted by the active signal transmission optical transceivers 70-1 and 71-1 is transmitted by the standby signal optical transceivers 80 and 81.
JP-A-11-340922 discloses the wavelength division multiplex optical transmission system as another technology, representing the method of performing the wavelength division multiplex transmission of the supervisory control signal through the waveform other than the one for the main signal (see FIG. 2). It is possible to use the transceiver with smaller information volume at the lower bit rate for the supervisory control signal compared with the main signal, and provide flexibility from the aspect of S/N (Signal to Noise) ratio compared with the main signal transmission.
A maintenance/management (OAM: Operation Administration Maintenance) function of the SONET/SDH and OTN has been enhanced by the use of the supervisory control signal for the transmission frame. Recently, the OAM function is insufficient for the original Ethernet®, and accordingly, the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) and IEEE (Institute of Electrical and Electronics Engineers) 802 Committee have promoted standardization for further enhancement.
Preferably, the multicore fiber is configured to have a uniform interval between geometrically adjacent cores for equally suppressing the optical crosstalk through transmission of the respective cores.
Arrangement of the cores will be described referring to FIGS. 3A to 3C. FIG. 3A is a sectional view of a generally employed single core fiber (hereinafter referred to as SCF) 1000. FIG. 3B is a sectional view of a 7-core MCF. FIG. 3C is a sectional view of a 19-core MCF. For the purpose of equalizing the interval between the cores of the 7-core MCF as shown in FIG. 3B, for example, respective cores 1510-1 to 1510-6 are arranged to form a regular hexagon having the core 1510-7 disposed at the center of the regular hexagon. As for the 19-core MCF as shown in FIG. 3C, the cores 1510-1 to 1510-12 are arranged at the respective apexes and medians of the regular hexagon. The cores 1510-13 to 1510-18 are arranged at the respective apexes of the regular hexagon with the common center, while having the core 1510-19 disposed at the center. The length of each side of the regular hexagon which is formed by the cores 1510-1 to 1510-12 is made twice as long as each section of the regular hexagon which is formed by the cores 1510-13 to 1510-18. This makes it possible to equalize the interval between the adjacent cores.
The MCF has a structurally difficulty in obtaining good optical coupling between the adjacent cores overall upon optical interconnection between the MCFs, which will be described referring to FIGS. 4A and 4B. FIGS. 4A and 4B are sectional views of the 7-core MCF which includes the peripherally arranged six cores (1510-1 to 1510-6) and the center core (1510-7). FIG. 4A represents an axial displacement which occurs in connection of the large-capacity optical fibers. It is assumed that the PC (Physical Contact) grinding used for the generally employed SCF is applied to the MCF optical interconnection. In this case, the possibility of displacement between the cores caused by the displacement in the rotary axis decreases at the center core. Meanwhile, the displacement in the rotary axis interferes with alignment of the cores, resulting in the loss between the outer peripheral cores. FIG. 4B illustrates the gap generated at the outer peripheral cores in connection to the large-capacity optical fibers. Likewise, the PC connection method is configured to grind the center of the fibers convexly and butting from both sides to realize appropriate optical interconnection. However, the MCF is likely to have the gap when the outer cores are brought into contact with each other (Please see the literature: Benjamin G. Lee, et al., “End-to-End Multicore Multimode Fiber Optic Link Operating up to 120 Gb/s”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 30, No. 6, p. 886, Mar. 15, 2012).